Echo signals from the field of fill-level measuring technology can differ from signals from other fields of application of echo-processing sensors. Apart from the generally unfavourable signal-to-noise ratios, deformation of echoes as a result of application influences, and the presence of false echoes play a role. An echo curve as recorded by a fill-level measuring device can comprise reflected signal components that overlap, or interfere with, the actual valid echo which is generated by reflection, for example on a smooth fill-level surface. Even within the antenna of a fill-level measuring device, reflected signal components can arise. The effect that a false echo is caused by the own antenna is known as antenna ringing.
In real filling good containers one or several locations of interference may be present. Examples of locations of interference in a filling good container include built-in pipelines, agitators or ladders. Such locations of interference can cause false echoes. If the location of interference does not perpendicularly extend to the direction of propagation of the measuring signal, then by superpositioning of reflections that can arise at many small positions of a location of interference a widened false echo can arise. However, for example during operation, the width of the false echo remains almost constant.
If the filling good container comprises a liquid as the medium to be measured, then an disturbed surface or an unsteady surface can arise, for example by an agitator. For reflection measuring, thereby conditions can result, which conditions are similar to those encountered in measuring bulk solids (bulk materials). By irregularly discharging or filling of the filling good container fluctuations over time can occur. The echo form resulting from this is difficult to predict and can change at any time during operation of the fill-level measuring device.
Furthermore, in particular in the case of bulk solids a deficient signal-to-noise ratio can render echo curve evaluation more difficult. There may be various reasons for the absence of a signal-to-noise ratio (signal-to-noise S/N). On the one hand a large measuring range can cause a small portion of reflected energy at a receiver or however, an inadequate focussing in the case of small antenna diameters can result in a poor signal-to-noise ratio.
Furthermore, it can happen that during operation a false echo is slowly overlapped by an echo caused by a filling good. By constructive or destructive interference of the carrier waves that are used by the fill-level measuring device, pumping of the echo amplitudes in the region of superposition can arise. This can lead to that, for example, temporary two echoes are visible, while a short time later only one single echo, but for example a long echo without amplitude bumps, is visible. This means that the valid echo disappears in the false echo. Furthermore, echo measuring in a fill-level measuring device can be made difficult in that the above-described effects can also occur in combination.
From the document WO 01/75474 a method for differentiating between ultrasound echoes that are reflected by different objects in a tank is known.
From the document U.S. Pat. No. 7,054,227 a method for generating an echo profile is known which comprises transmitting transmission bursts of energy onto a reflective surface, and which method comprises carrying out recursive descent parsing of the echo profile.
From the document U.S. Pat. No. 5,956,663 a process control system having a sensor input, a sensor diagnosis circuit and a wavelet pre-processing circuit is known.
Furthermore, in the document U.S. Pat. No. 6,097,669 a method for wavelet filtering of a sodar signal is described.
Moreover, from US 2005/0066731 a method for generating an echo signal is known, which method comprises transmitting one or several bursts of energy onto a surface, receiving reflected pulses from the surface, converting the reflected pulses to an echo signal that comprises one or several potential echo pulses, applying an entropy filter on the echo signal, and differentiating the echo pulses that have been determined to comprise noise in the echo signal.
Furthermore, wavelets are known from the book Wavelets—Eine Einführung für Ingenieure, Oldenburg Verlag, 2002, Werner Bäni.
Furthermore, from the book Kristian Kroschel, Statistische Informationstechnik, 4th edition, Springer Verlag, 2004, modern methods of signal rerepresentation such as the wavelet transformation are known.
Moreover, from the book Bernd Jähne, Digitale Bildverarbeitung, 6th edition, Springer Verlag, 2005, the fast calculation of unitary transformations is known.
Furthermore, from Michel Misiti et al., Wavelet Toolbox 4 User's Guide, The Mathworks, Inc., 2007, scale aspects of wavelets are known.
Furthermore, from Samuel S. Blackman, Multiple-Target Tracking with Radar Applications, Artech House, 1986, fundamentals of multiple-target tracking are known.
Moreover, from EP 0 882 956 a method for measuring the fill level of a feed material in a container according to the radar principle is known.
Furthermore, from DE 42 34 300 A1 a fill-level measuring method is known, in which repeatedly in a transmission phase at least one pulse is emitted in the direction of the filling good surface to be detected.
Furthermore, U.S. Pat. No. 5,157,639 discloses a detector comprising a transducer, an A/D converter, gate generation means, a storage device, determination means and signal generation means.
Furthermore, from DE 10 2005 063 079 a method for determining and monitoring the fill level of a medium in a container according to a run-time measuring method is known.
When measuring a fill level an echo function or echo curve may arise. For further processing, this echo function is often digitalised with methods of the signal technology, and is presented in digital form. Depending on the selected resolution of the digitalised echo function, a plurality of digital data may arise. This plurality of digital data is, for example, to be transmitted to an evaluation device via a digital bus system, or the plurality of data is to be stored for subsequent further processing.
The echo function can be analysed for echoes by means of an echo extraction method. Due to an unsteady progression a real echo curve may lead to inaccurate results in determining the position of echoes, and consequently misinterpretation in the determination of echoes may occur.
Furthermore, the echo function is often digitalised, for further processing, with methods of signal technology, and is presented in digital form. Due to different reflections the progression of the echo function can be unsteady, in other words jagged. The unsteady progression of the echo function can make it difficult to find an echo in an echo function.
The echo function can comprise echoes from whose position the fill level in a container can be determined. However, the echo function can comprise a function progression that is similar to that of an echo but that has not been caused by a relevant echo. In conventional methods for echo determination from an echo function, such a region, which is difficult to distinguish from a true echo, may result in instances of misinterpretation. Misinterpretation may, however, mean that an echo is detected at a position at which there is no relevant echo. Since the position of the echo is used to determine the fill level, misinterpretation of the position of the echo may lead to an incorrect determination of the fill level.
For further processing of the current echo function, all the current echoes or potential echoes that are contained in the current echo function may be found and provided in an echo list, in particular a multiple echo list. Depending on the algorithm used for finding the current echoes, the multiple echo list may also comprise current echoes that do not correspond to any relevant echo or true echo. During evaluation of the current echoes that are stored in the multiple echo list it may not be possible to unambiguously identify a current echo as a relevant echo. In the interpretation of the current echoes from the echo list, instances of misinterpretation may occur.
The echo function can comprise a plurality of echoes, wherein based on the determination of the position of echoes a statement relating to the height of a fill level in a container is possible. A real threshold value curve may, however, due to undesired reflections comprise an unsteady progression, as a result of which the determination of the position of an echo may turn out to be incorrect, and as a result of which a provided statement relating to the height of a fill level may contain errors.